1. Field of the Invention
This invention relates generally to frequency converters, and relates more particularly to a self-oscillating mixer with Darlington-connected transistors and a feedback resonator.
2. Description of the Relevant Art
Frequency converters are used to change the frequency of a signal to a different frequency, without loss of the information contained in the signal. They are often used in communications and radar systems, wherein, for example, a received signal is down-converted to a lower frequency for amplification, filtering, and other signal processing.
Frequency converters typically consist of a local oscillator, which generates a signal at a reference frequency, and a mixer, which combines an input signal and the local-oscillator signal to generate an output signal. The output signal contains the frequency products of the input and local-oscillator signals, which are the sums and differences of the input and local-oscillator signals and multiples thereof. Most commonly, a down conversion is desired, in which case the frequency product equal to the difference between the local-oscillator frequency and the input frequency is allowed to pass through a band-pass filter, and the other frequency products are filtered out.
Some mixers used in frequency converters are passive devices, such as diode mixers, which may utilize, for example, one or several diodes. Since diode mixers are passive devices, no dc current is required to power them. The local-oscillator signal provides the power, but at a cost that may be greater than if dc current powered the diode mixer. Also since diode mixers are passive devices, there is a loss in signal strength through the mixer, so that the output signal is weaker than the input signal. Most frequency converters using diode mixers require a separate local-oscillator device to generate the local-oscillator signal.
Other mixers used in frequency converters are active devices using transistors to provide some signal gain. At microwave frequencies, active mixers usually are designed around gallium arsenide field-effect transistors. Frequency converters using these active mixers also require a separate local-oscillator device to generate the local-oscillator signal.
One type of frequency converter, known as a self-oscillating mixer, does not require a separate local-oscillator device because the self-oscillating mixer generates its own local-oscillator signal. Self-oscillating mixers are two port devices, input and output, in contrast to the three port mixers described above, which have a local-oscillator port in addition to their input and output ports. Single bipolar transistors have been used in self-oscillating mixers operating in the radio frequency range. However, self-oscillating mixers using single bipolar transistors do not operate acceptably at the upper range of microwave frequencies because of low input impedance, which causes signal reflections, and because of low gain at the high frequencies. In addition, single transistor self-oscillating mixers suffer from the disadvantage that one degree of design freedom is lost due to the combining of the mixer and local-oscillator devices.
For frequency conversion at microwave frequencies, self-oscillating mixers fabricated from gallium arsenide field-effect transistors have been used, including both single and dual gate devices. One disadvantage to such mixers is the expense of gallium arsenide devices.